The present invention relates in general to implantable cardiac stimulation devices, including bradycardia and antitachycardia pacemakers, defibrillators, cardioverters and combinations thereof that are capable of measuring physiological data and parametric data pertaining to implantable medical devices. Particularly, this invention relates to a system and method for automating detection of atrial capture in an implantable cardiac stimulation device using far-field signal detection. More specifically, this invention provides for the automatic optimization of far-field R-wave sensing by switching electrode polarity during atrial capture verification.
Implantable medical devices, such as pacemakers, defibrillators, and cardioverters (collectively referred to herein as implantable cardiac stimulating devices), are designed to monitor and stimulate the heart of a patient that suffers from a cardiac arrhythmia. Using leads connected to a patient""s heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart""s regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program (which is responsible for the operation of the implantable device) can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Modern programmable pacemakers, the most commonly used implantable devices, are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart (e.g. either the right ventricle or the right atrium). In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart (e.g. both the right atrium and the right ventricle). The left atrium and left ventricle can also be paced, provided that suitable electrical contacts are effected therewith.
In general, both single and dual-chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e. the chamber where a stimulation pulse is delivered)xe2x80x94with a xe2x80x9cVxe2x80x9d indicating the ventricle, an xe2x80x9cAxe2x80x9d indicating the atrium, and a xe2x80x9cDxe2x80x9d (dual) indicating both the atrium and ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and where an xe2x80x9cOxe2x80x9d indicates that no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (xe2x80x9cIxe2x80x9d) response, where a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger (xe2x80x9cTxe2x80x9d) response, where a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event; or (3) a Dual (xe2x80x9cDxe2x80x9d) response, where both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, xe2x80x9cRxe2x80x9d, is sometimes added to the code to signify that the particular mode identified by the three letter code is rate-responsive, where the pacing rate may be adjusted automatically by the pacemaker based on one or more physiological factors such as blood oxygen level or the patient""s activity level.
Modem pacemakers also have a great number of adjustable parameters that must be tailored to a particular patient""s therapeutic needs. One adjustable parameter of particular importance in pacemakers is the pacemaker""s stimulation energy. xe2x80x9cCapturexe2x80x9d is defined as a cardiac response to a pacemaker stimulation pulse. When a pacemaker stimulation pulse stimulates either a heart atrium or a heart ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a xe2x80x9ccapture thresholdxe2x80x9d which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should be achieved at the lowest possible energy setting yet provide enough of a safety margin so that, should a patient""s threshold increase, the output of an implantable pacemaker, i.e. the stimulation energy, will still be sufficient to maintain capture. Dual-chamber pacemakers may have differing atrial and ventricular stimulation energy that correspond to atrial and ventricular capture thresholds, respectively.
The earliest pacemakers had a predetermined and unchangeable stimulation energy, which proved to be problematic because the capture threshold is not a static value and may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around pacemaker lead heads within several months after implantation may raise the capture threshold.
As a result, some patients eventually suffered from loss of capture as their pacemakers were unable to adjust the pre-set stimulation energy to match the changed capture thresholds. One attempted solution was to set the level of stimulation pulses fairly high so as to avoid loss of capture due to a change in the capture threshold. However, this approach resulted in some discomfort to patients who were forced to endure unnecessarily high levels of cardiac stimulation. Furthermore, such stimulation pulses consumed extra battery resources, thus shortening the useful life of the pacemaker.
When programmable pacemakers were developed, the stimulation energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted pacemaker via a magnet applied to a patient""s chest or via telemetry. The particular setting for the pacemaker""s stimulation energy was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient""s capture threshold, from the patient""s medical history, and from a listing of the patient""s medications. While the adjustable pacing energy feature proved to be superior to the previously known fixed energy, some significant problems remained unsolved. In particular, when a patient""s capture threshold changed, the patient was forced to visit the medical practitioner to adjust the pacing energy accordingly.
To address this pressing problem, pacemaker manufacturers have developed advanced pacemakers that are capable of determining a patient""s capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This approach, called xe2x80x9cautocapturexe2x80x9d, improves the patient""s comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the pacemakers battery life by conserving the energy used to generate stimulation pulses.
However, many of these advanced pacemakers require additional circuitry and/or special sensors that must be dedicated to capture verification. This requirement increases the complexity of the pacemaker system and reduces the precious space available within a pacemaker""s casing, and also increases the pacemaker""s cost. As a result, pacemaker manufacturers have attempted to develop automatic capture verification techniques that may be implemented in a typical programmable pacemaker without requiring additional circuitry or special dedicated sensors.
A common technique used to determine whether capture has been effected is monitoring the patient""s cardiac activity and searching for the presence of an xe2x80x9cevoked responsexe2x80x9d following a stimulation pulse. The evoked response is the response of the heart to application of a stimulation pulse. The patient""s heart activity is typically monitored by the pacemaker by keeping track of the stimulation pulses delivered to the heart and examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue (myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by generation of an R-wave (sometimes referred to as the xe2x80x9cQRSxe2x80x9d complex).
When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the atrium (hereinafter referred to as an A-pulse), any response sensed by atrial sensing circuits of the pacemaker immediately following application of the A-pulse is presumed to be an evoked response that evidences capture of the atria.
However, it is for several reasons very difficult to detect a true evoked response. First, because the atrial evoked response is a relatively small signal, it may be obscured by a high energy A-pulse and therefore difficult to detect and identify. Second, the signal sensed by the pacemaker""s sensing circuitry immediately following the application of a stimulation pulse may be not an evoked response but noisexe2x80x94either electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as an A-pulse, across the interface. Unfortunately, because the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to herein as an xe2x80x9cafterpotentialxe2x80x9d, formed at the electrode can corrupt the evoked response that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time. In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn leads to missed heartbeatsxe2x80x94a highly undesirable and potentially life-threatening situation. Another problem results from a failure by the pacemaker to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact presentxe2x80x94also an undesirable situation that will cause the pacemaker to unnecessarily invoke the pacing energy determination function in a chamber of the heart.
Automatic pacing energy determination is only invoked by the pacemaker when loss of atrial or ventricular capture is detected. An exemplary prior art automatic atrial pacing energy determination procedure is performed as follows. When loss of atrial capture is detected, the pacemaker increases the A-pulse output level to a relatively high predetermined testing level at which capture is certain to occur, and thereafter decrements the output level until atrial capture is lost. The atrial pacing energy is then set to a level slightly above the lowest output level at which atrial capture was attained. Thus, atrial capture verification is of utmost importance in proper determination of the atrial pacing energy.
When an atrial stimulation pulse is properly captured in the atrium, a subsequent ventricular contraction results in an R-wave which may be sensed through an atrial lead, in patients with intact atriovenricular (xe2x80x9cAVxe2x80x9d) conduction, as a xe2x80x9cfar-fieldxe2x80x9d signal. The far-field R-wave confirms successful atrial capture because the ventricular contraction only occurs after a properly captured atrial stimulation pulse. Previously known pacemakers have ignored this useful phenomenon because previously known single-chamber atrial pacemakers and dual-chamber pacemakers programmed to operate in an atrial mode purposefully do not sense ventricular activity through the atrial lead for a particular period of time (i.e. the xe2x80x9crefractoryxe2x80x9d period) after delivery of the atrial stimulation pulse. Furthermore, the polarization signal formed at the atrial lead electrode may obscure and/or distort the far-field R-wave signal, even if it were sensed.
A further difficulty in achieving optimal sensing of desired signals is selecting the most appropriate electrode polarity configuration. Typically, either a unipolar or a bipolar configuration is used for pacing and sensing in the heart chambers.
In a unipolar configuration, one electrode is positioned at, or near the distal end of the lead body, in contact with the heart tissue. A ground or xe2x80x9cindifferentxe2x80x9d electrode, commonly the pacemaker housing or can, is placed some distance away. In a bipolar configuration, two electrodes are placed in close proximity to each other at the distal end of the lead body, typically in a xe2x80x9ctipxe2x80x9d and xe2x80x9cringxe2x80x9d configuration, such that both electrodes have contact with the heart tissue.
Determining the ideal polarity configuration remains enigmatic. Medical practitioners tend to have personal preferences and patient variability may make one configuration more successful than another for unknown reasons. Generally, bipolar configurations require less pacing energy, and are less prone to noise or crosstalk than unipolar configurations. Crosstalk is defined as the sensing of signals occurring in other heart chambers, sensing output from other channels in a multi-chamber device, or from other devices when more than one stimulating device is implanted. Noise signals can occur when myopotentials are detected by the lead system. Bipolar pacing is preferred over unipolar pacing when extraneous stimulation of skeletal muscle tissue occurs or device pocket infection occurs. However, unipolar pacing and sensing also present certain advantages. Compared to bipolar configurations, greater sensitivity is achieved and polarization effects are lessened due to a typically large indifferent electrode. Sensing in the atrium may be better achieved by unipolar sensing configurations since P-wave signals are relatively small in amplitude. Particular tasks in detecting sensed events in response to a stimulation pulse may also be better performed in unipolar systems.
New combinations of electrodes are now available, widening the selection a physician has to choose from in deciding which configuration is the most suitable. For example, unipolar systems may be selectively programmable as using a lead tip electrode and pacemaker can or a lead ring electrode and pacemaker can. Combipolar systems using the lead tip electrodes or lead ring electrodes of two different leads, that is xe2x80x9ctip-to-tipxe2x80x9d or xe2x80x9cring-to-ringxe2x80x9d configurations, are also possible in dual chamber devices.
In light of these new combinations and the complexity of pacing systems which may be sensing and pacing in up to four heart chambers, it is desirable to allow selection of the electrode polarity that works best in both minimizing pacing energy and accurately sensing intrinsic as well as evoked responses following stimulation pulses.
Methods of automatically switching electrode polarity for attaining optimal sensing or pacing configurations for a given task or under specific circumstances are known in the field. Reference is made to U.S. Pat. No. 4,549,548 to Wittkampf et al.
Despite these methods for improving the sensing capabilities of a pacemaker, there remains an unsatisfied need for accurately verifying atrial capture based on the sensed signals. It would thus be desirable to provide a system and method for enabling the pacemaker to automatically and accurately perform atrial capture verification by sensing and identifying a far-field R-wave that occurs only after delivery by the pacemaker of a successfully captured atrial stimulation pulse. It would also be desirable to provide a system and method for reducing the negative effect of polarization and noise on capture verification by automatically isolating such negative effects from the identified far-field R-wave signal. It would further be desirable to allow automatic electrode polarity configuration switching during atrial capture verification such that sensing of far-field R-waves is optimized. It would further be desirable to enable the pacemaker to perform atrial capture verification without requiring dedicated circuitry and/or special sensors.
The disadvantages and limitations discussed above are overcome by the present invention. In accordance with the invention, a system and method are provided for automating verification of proper atrial capture of pacing pulses generated by a patient""s implantable cardiac stimulation device by sensing and identifying a far-field ventricular signal resulting from a ventricular contraction that follows a successfully captured atrial stimulation pulse. The system and method of the present invention compensate for effects of polarization and noise on the identified far-field signal and do not require use of special dedicated circuitry or special sensors to implement the automated procedure. All of the aforesaid advantages and features are achieved without incurring any substantial relative disadvantage.
The present invention provides an implantable medical device (hereinafter xe2x80x9cpacemakerxe2x80x9d) equipped with cardiac data acquisition capabilities. A preferred embodiment of the pacemaker of the present invention includes a control system for controlling the operation of the pacemaker, a set of leads for receiving atrial and ventricular signals and for delivering atrial and ventricular stimulation pulses, a set of sense amplifiers for sensing and amplifying the atrial and ventricular signals, a sampler, such as an A/D converter, for sampling atrial and/or ventricular signals, and pulse generators for generating the atrial and ventricular stimulation pulses. In addition, the pacemaker includes memory for storing operational parameters for the control system, such as atrial or ventricular signal sampling parameters, and atrial or ventricular signal samples. The pacemaker also includes a telemetry circuit for communicating with an external programmer.
In a preferred embodiment of the invention, the pacemaker control system periodically performs an atrial capture verification test and, when necessary, an atrial pacing threshold assessment test, which performs an assessment of the stimulation energy in the atrial chamber of the patient""s heart. The frequency with which these tests are performed are preferably programmable parameters set by the medical practitioner using an external programmer when the patient is examined during an office visit or remotely via a telecommunication link. The appropriate testing frequency parameter will vary from patient to patient and depend on a number of physiologic and other factors. For example, if a patient is on a cardiac medication regimen, the patient""s atrial capture threshold may fluctuate, thus requiring relatively frequent testing and adjustment of the atrial stimulation energy. Preferably the system and method of the present invention are implemented in a pacemaker operating in an atrial mode such as AAI, AOO or AAT.
In a first embodiment of the invention, the pacemaker delivers an atrial stimulation pulse and then samples a resulting far-field ventricular signal during a predetermined far-field interval window that is centered at the expiration of a predetermined window delay. The pacemaker then compares the far-field signal sample to a predetermined far-field signal recognition template to verify whether the far-field signal sample morphology corresponds to a far-field R-wave that is expected to follow a successfully captured atrial stimulation pulse. If the far-field signal sample is approximately equal to the far-field signal recognition template, then atrial capture is deemed verified. Otherwise, the pacemaker performs an atrial stimulation energy determination procedure. This embodiment of the invention is preferably implemented in a pacemaker that is equipped with special electrodes and/or circuitry for reducing or eliminating noise and polarization signals that occur after delivery of atrial stimulation pulses.
In a second embodiment of the invention, the pacemaker delivers an atrial stimulation pulse, samples a response signal in the atrium, and then samples a resulting far-field ventricular signal during a predetermined far-field interval window that is centered at the expiration of a predetermined window delay. The response sample is then compared to the far-field sample and is compared to a predetermined far-field signal recognition template. If the sample is approximately equal to the far-field signal recognition template, then atrial capture is deemed verified. Otherwise, the pacemaker performs an atrial stimulation energy determination procedure.
Preferably, the window delay and the far-field signal recognition template are automatically determined by the pacemaker after initial implantation, and updated at other times as necessary or appropriate, as for example under the direction of the medical practitioner during a follow-up visit. In accordance with the invention, the pacemaker performs an AR conduction test to determine a conduction time and then stores the conduction time in memory. The pacemaker then delivers an atrial stimulation pulse, samples a response signal in the atrium, stores the response sample in memory, then samples a resulting far-field ventricular signal after a delay approximately equal to the conduction time and stores the far-field sample in memory.
When a predetermined number of samples and conduction times are thus acquired, the pacemaker averages each set of samples and subtracts the response sample average from the far-field signal sample average to produce a far-field signal recognition template, which is then stored in memory. The pacemaker also averages the conduction times to determine an average window delay, centers the predefined far-field interval window at the average conduction time (window delay) and stores the position of the far-field interval window in memory.
Alternately, the window delay and the far-field signal recognition template may be predefined by the medical practitioner and stored in the pacemaker memory along with the far-field interval window.
In an alternative embodiment, the present invention provides for automatic electrode polarity switching during the atrial capture verification method. In systems using a bipolar sensing configuration in the atrium, far-field signals may or may not be detected due to the characteristically low amplitude of these signals. In such bipolar systems, polarity switching, that is switching from bipolar sensing to unipolar sensing, at the onset of a far-field interval window will be advantageous in detecting far-field R-waves for verification of capture. Thus, an optional programmable feature is provided that will enable or disable automatic switching to unipolar sensing at the onset of the far-field interval window and switching again back to bipolar pacing at the end of the far-field interval window.
The system and method of the present invention thus automatically verify atrial capture and, when necessary, automatically determine a proper atrial stimulation energy of the patient""s pacemaker, without requiring dedicated or special circuitry and/or sensors.